فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri - Paris Observatory

A comet having a short period of 6.62 years discovered by Biela.
It broke up on its 1846 return and subsequently gave rise to a
spectacular meteor shower.

In honor of Wilhelm von Biela (1782-1856), Austrian military
officer and amateur astronomer, who re-discovered the comet
Biela in 1826, although it had been seen first in 1772.
→ comet.

comet

دنباله‌دار، دمدار

donbâledâr (#), domdâr (#)

Fr.: comète

A small body of → gas and → dust
which revolves around the → Sun
in a usually very → elliptical or even
→ parabolic → orbit.
It is seen to be composed of a → head,
or → coma, and often with a spectacular gaseous
→ tail extending a great distance from the head. The rocky-icy
head is called the → comet nucleus. As the comet nears the Sun,
the increased temperature causes the → ice
in the nucleus to → sublimate
and form a gaseous halo around the nucleus, called the coma. Comets often possess
two tails, a → dust tail that lies in the orbit behind
the comet generated by surface activity, and a brighter, ionized
→ gas tail, that points away from the Sun, driven by
→ solar wind. → Long-period comets
are thought to originate in the → Oort cloud,
at distances exceeding 50,000 → astronomical units (AU).
They are perturbed by the planets (especially → Jupiter)
to fall in toward the Sun. Their orbits typically have random inclinations and a very large
→ eccentricity; some
→ hyperbolic orbits have been observed.
→ Short-period comets
apparently arise in the → Kuiper belt in the zone from 20 to 50
AU. Their orbits typically have small eccentricities. Both cometary reservoirs
are thought to represent primordial solar system material. A comet
with a dust coating on its surface that inhibits gas production might be classified as an
→ asteroid. Because of this ambiguity, objects such as
→ Chiron, a → Centaur asteroid,
have been reclassified as comets.
Comets are primarily composed of amorphous → water
ice, but also contain
→ carbon dioxide (CO2),
→ carbon monoxide (CO),
→ formaldehyde (H2CO),
→ methanol (CH3OH),
→ methane (CH4) at a few percent level (with
respect to water), and many other molecules at a lower level. See also
→ comet designation.

From O.Fr. comète, from L. cometa, from Gk. (aster) kometes,
"long-haired (star)," from kome "hair of the head," so called from
resemblance of the comet's tail to streaming hair.

A comet with an irregular → nucleus of roughly 3 x 5 km
across orbiting the Sun between → Jupiter and
→ Earth
with a period of 6.45 years. Officially named 67P/Churyumov-Gerasimenko,
the comet has been observed from Earth on seven approaches to the
Sun: in 1969, 1976, 1982, 1989, 1996, 2002, and 2009. It was also
imaged by the → Hubble Space Telescope
in 2003, which allowed estimates
of its size and shape. It will arrive at → perihelion
on 13 August 2015. In 2014 the → European Space Agency
probe → Rosetta, launched in 2004, will be placed on an orbit around
67P/Churyumov-Gerasimenko. Over an entire year, as it approaches the Sun, Rosetta will
map the comet's surface and study changes in its activity.

→ comet;
Named after its discoverers, Klim Churyumov and Svetlana Gerasimenko, Ukrainian
astronomers, who first noticed the comet in 1969.

comet designation

نامگزینی ِ دنباله‌دار

nâmgozini-ye donbâledâr

Fr.: désignation des comètes

A → nomenclature system for naming
→ comets.
In early 1995, a new comet designation system was established by the
→ International Astronomical Union. The main rules
are as follows: a) If the comet is a newly discovered one, it first gets a provisional name,
which closely matches the → asteroid designation
system. For example, the first comet discovered in
the first half of 1998 January is designated 1998 A1, the second 1998 A2,
etc. b) The name of the person(s) who discovered the comet may
be added to this designation (limited, however, to three names). For example, comet
→ Hale-Bopp (C/1995 O1) has its full name as Hale-Bopp C/1995 O1,
whereas its designation is C/1998 O1. If several people are involved with a
discovery at an observatory, the comet may be named after the observatory instead of the
individuals. c) → Long-period comets and one-apparition
→ periodic comets receive
only a provisional designation. d) A → short-period comet would get the
P/designation until it is recovered in a second → apparition.
At this point, the
P/Year designation would be replaced with a number followed immediately by an upper case P,
and a slash followed by the name of the discoverer(s). The number here is
one more than the number of known periodic comets that have reappeared. For example,
the comet
Hug-Bell (P/1999 X1) was given the full name 178P/Hug-Bell after it reappeared in
2007. Previously, 177 periodic comets had got assigned numbers. e) Long-period comets are indicated by the prefix C. f) If the comet is destroyed, or if it fails to appear after several
apparitions, it would be prefixed D/
(→ defunct comet) followed by the year of
its discovery. For example, → Comet Shoemaker-Levy 9
has been assigned D/1993 F2 since it was discovered in the second half of March in 1993
and was destroyed when it crashed into Jupiter in 1994. g) Comets that lack sufficient position
measurements for an orbital determination are given the designation of X/ followed
by the year of their discovery and the appropriate letter and number code. h) When a → comet nucleus
nucleus splits, each fragment
is given the comet designation followed by A, B, C, etc (for fragments).

One of the brightest comets seen in the twentieth century, even though
it came no closer to Earth than 1.32 AU (on 22 March 1997). It was
visible to the naked eye for many months. The → nucleus
of Hale-Bopp was estimated to be about 30 to 40 km across. Hale-Bopp has an orbital
period of 2,380 years and is predicted to be seen again in AD 4377.

A → long-period comet found in January 1996, which
became the brightest comet since → Comet West
in 1976. It was a bright naked-eye object and remained so in
March, April, and May of 1996.
At closest approach to Earth on March 25, is was only 0.10 AU away,
displaying a long tail of up to 100 degrees. Small fragments were observed
to break off the main nucleus. Hyakutake was the first comet from
which X-ray emission was detected.

→ comet;
Named after the Japanese amateur astronomer Yuuji Hyakutake (1951-2002), who discovered
this comet in the morning of January 30, 1996.

comet nucleus

هسته‌ی ِ دنباله‌دار

haste-ye donbâledâr (#)

Fr.: noyau de comète

The solid, centrally located part of a → comet.
The nucleus is a mass of dust and frozen gases. When
heated by the → Sun,
the gases sublimate and produce an atmosphere
surrounding the nucleus known as the → coma,
which is later swept into an elongated tail.
Reliable measurements of cometary nuclei indicate sizes from a few km to 10 or 20 km.
The nucleus of → Comet Hale-Bopp
is one of the largest (perhaps 40 km).
The composition of the nucleus is determined by measuring the
composition of the coma (except for 67P/Churyumov-Gerasimenko).
The dominant → volatile is → water,
followed by → carbon dioxide (CO2),
→ carbon monoxide (CO),
→ formaldehyde (H2CO),
→ methanol (CH3OH),
→ methane (CH4) at a few percent level (with
respect to water) and many other molecules at a lower level.

A comet, formally designated D/1993 F2, whose
shattered nucleus crashed into the planet → Jupiter
over the period of July 16-22, 1994,
several months after its discovery. The collision produced scars that were visible
from Earth even in small telescopes. The cataclysmic event was the first collision between
two → solar system
bodies ever observed. The comet had been discovered
on March 24, 1993, from photographs taken
using the 0.46 m → Schmidt telescope at
→ Palomar Observatory.
The appearance of the comet was reported as "most unusual": the
object appeared as a "dense linear bar'' with a "fainter, wispy
tail.'' The comet's brightness was reported as about magnitude 14,
more than a thousand times too faint to be seen with the naked
eye. Later observations revealed that the "bar'' was made up of as
many as 21 pieces "strung out like pearls on a string,'' according to
one researcher.
Orbit calculations show that on July 7, 1992, the comet had passed only 25,000 km
above Jupiter. The differential pull of the planet's enormous
→ gravitational force
on the near and far sides of the comet fragmented
it into the 21 or more pieces with sizes estimated at up to 2 km
and an enormous amount of smaller
debris. The comet had been in a rapidly changing orbit around Jupiter for
some time before this, probably for at least several decades.

→ comet;
Named after the husband and wife scientific team of American Carolyn S. (1929-) and
Eugene M. Shoemaker (1928-1997)
and Canadian amateur astronomer David H. Levy (1948-)

Comet West (C/1975 V1)

دنباله‌دار ِ وست، دمدار ِ ~

donbâledâr-e West, domdâr-e ~

Fr.: comète West

A spectacular comet that at its closest approach to Earth reached a brightness
of -1 magnitude. It was so bright that could be seen even at sunrise. The comet reached
→ perihelion on 1976 Feb. 25 at 0.20 A.U. and had a
fan-shaped tail of dimensions 25° x 25° x 15° on the sky. A few days after perihelion,
the nucleus split in four fragments. The → carbon monoxide (CO)
molecule in comets was first detected in West. The comet's orbit has a period of
about 500,000 years. Formerly designated 1976 VI.

The appearance of → gas and → dust
features from the rocky-icy nucleus of a
comet when approaching the Sun (→ cometary atmosphere,
→ cometary tail).
The → sublimation of → water
can explain cometary activity at distances
from the Sun up to about 4 → astronomical units.
At larger distances, the average
temperature of the → comet nucleus'
surface is less than 140 K, too low for
efficient sublimation of water → ice.
However, there are many examples
of cometary activity at larger distances.
This can probably be due to the sublimation of more
→ volatile → chemical species.
Indeed, radio spectroscopic observations of comets at large
distances have revealed an important → outgassing
of → carbon monoxide (CO), which
can sublimate at temperatures as low as 25 K.

A relatively small cloud of → dust and
→ gas in the → interstellar medium
shaped like a comet with a bright-rimmed head. Cometary globules are situated near
young → massive stars with a strong
→ stellar wind. The wind ionizes gases
on the side facing the → O stars and sweep away the low-density
gas toward the tail. Cometary globules are believed to be
→ molecular cloud
condensations, which are so dense that they are not disrupted when an
→ H II region expands into the molecular cloud surrounding it.
The → Rosette nebula is a good example of an
H II region which shows an abundance of cometary globules.

The → path followed by a → comet in the
→ solar system around the → Sun.
Most cometary orbits appear to be → elliptical,
or in some cases → parabolic.
The orbits of → short-period comets are
elliptical, carrying them out to a region lying from → Jupiter
to beyond the orbit of → Neptune.
Those of → long-period comets are
very elliptical. The orbits may be strongly influenced if they pass near the Jovian
planets, particularly Jupiter itself.
The cometary orbits are also influenced to some degree by gases shooting out
of comets, so their orbits are primarily but not completely determined
by gravity.
Newton (1644-1727) was the first to
compute a cometary orbit. He found that the comet of 1680 was following a
parabolic orbit around the Sun. Edmond Halley (1656-1742),
following the methods of Newton, computed the → orbital elements
of 24 comets. He realized that the comets of 1531, 1607 and
1682 had very similar elements and postulated that they were in fact
the same object, orbiting an elongated ellipse. He predicted the next
return to occur in 1758 or early 1759. The return of what is now
called Halley's comet was observed after his death, This first
observation of a "predicted" comet is manifestly one of the major
successes of → celestial mechanics.

A device consisting of two vertical coaxial cylinders and a fluid filling the
volume between the cylinders and used for measuring the viscosity of the fluid.
The inner cylinder is stationary while the outer cylinder rotates. The amount of
shear stress produced owing to rotation is directly proportional to
the viscosity of the fluid.

A comet that has lost its ability to emit dust and gas and no longer displays
the classic cometary features of a nebulous coma and a tail, or a destroyed comet.
Comet Pigott
The asteroid (944) Hidalgo is considered by some to be a defunct comet because
of its unusual orbit. Another case is the asteroid Phaethon.

A faint comet with the shortest known period (about 3.30 years). Its semimajor axis is
2.21 AU and aphelion 4.1 AU. it is the parent body of the Taurids meteor shower.
The comet was first observed in 1786 by the French astronomer Pierre Méchain.

Named after the German astronomer Johann Franz Encke (1791-1865), who in 1819 computed
its orbit and proved that sightings of apparently different comets in 1786, 1795, 1805,
and 1818 were in fact appearances of the same comet. → comet.